Infra-red radiation consists of electromagnetic radiation in a band of wavelengths extending from about 750 nanometers to 1 millimeters. Of particular interest is infra-red radiation with a wavelength of about 10.6 micrometers since such radiation can readily be produced by a carbon dioxide laser. Moreover, a typical carbon dioxide laser can generate 10.6 micrometers, infra-red radiation at power levels of tens of watts on a continuous basis. A number of applications exist for such levels of continuous infra-red energy, provided the energy can be efficiently concentrated and directed. One such application of particular interest is in the field of medicine, where a concentrated and directed beam of infra-red energy can be used as a cutting instrument in place of a conventional scalpel. If the beam of infra-red energy can be reliably directed through an endoscope, internal surgery becomes possible in circumstances where a conventional scalpel could not be employed.
The bulk and weight of a carbon dioxide laser for powering an infra-red "scalpel" are such that the laser must remain outside the body being operated upon, and therefore some form of infra-red energy guide is necessary to link the laser output to the location of surgical operations within the body. Fiber optic endoscopes are known for visual inspections of the interior of the human body, and require a length which may be in excess of 20 centimeters, with an approximately circular cross-section of extremely limited dimensions, usually only a few millimeters. However, equivalent fiber optic endoscopes for use at infra-red wavelengths are not available owing to the lack of optically efficient materials. Certain materials which are optically suitable, such as zinc selenide, are toxic and therefore excluded from medical applications.
Proposals have been made for transmission of infra-red energy down a succession of straight hollow tubes linked by pivot systems, with each pivot system containing one or more mirrors to deflect an infra-red beam from one tube into the next, the transmission system possibly also containing one or more focussing lenses. Apart from difficulties of achieving correct alignment, optical inefficiencies (particularly due to the need for multiple mirrors) generally result in the unacceptably high loss along the transmission system of about half of the initial infra-red energy. Also, no such system is available which matches the dimensional limitations or flexibility requirements of an endoscope.
U.S. Pat. No. 4,068,920 describes a flexible hollow rectangular waveguide for transmission of infra-red radiation. Although the waveguide of U.S. Pat. No. 4,068,920 is a relatively efficient transmitter of infra-red energy, its extreme width-to-thickness ratio clearly renders it impracticable for use within the circular body of an endoscope - see column 4, lines 20-47. Preferred dimensions and forms of construction also render this rectangular waveguide unsuitable for in-body surgery from the considerations of maintaining scrupulous cleanliness and an ability to be thoroughly sterilized. A very brief and undetailed reference at the end of column 2 to use for surgery therefore inevitably refers to external surgery (i.e. cutting the body open from the outside with exactly the same techniques as used with a conventional manual scalpel), since there is no teaching of internal use, nor any suggestion as to how the waveguide could be adapted to endosocopic surgery. A circular infra-red guide of suitable dimensions could be fitted inside an endoscope sheath, but lines 31-36 in column 1 of U.S. Pat. No. 4,068,920 clearly state that such circular waveguides are not flexible and cannot be used in a flexible application because of the large bending losses (of infra-red energy). The prior art thus indicated that flexible infra-red guides suitable for use in endoscopes were not known and not considered to be possible.